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Area sensor but line scan


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#1 Roberto Pirodda

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Posted 01 August 2012 - 03:53 AM

The best scanning method is via a line scan camera. Now if we have an area scan sensor but with line scan technique we could avoid all drawback of mechanical shifting. Is it possible? via software ?
Rob
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#2 Dirk DeJonghe

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Posted 01 August 2012 - 11:42 AM

I fail to see a significant difference between line-scan and area-scan. In case of line-scan you have to move the film mechanically during the exposure with three different linearrays in place, with an area sensor, the film can be held firm on a shuttle while the successive red, green and blue exposures take place, each pixel falling on exactly the same place in the sensor.
I have compared several different scanners: Oxberry, Northlight, Arriscan, Director, Spirit4K, Scanity.
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#3 Roberto Pirodda

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Posted 01 August 2012 - 12:49 PM

well, my idea was exactly to avoid mechanical shifting because i am building my own scanner with 3 RGB filters ( so 3 exposures for each frame) and i am not sure that even the best scanner ever can be so mechanically perfect at pixel level ( correct me if i am wrong).
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#4 Robert Houllahan

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Posted 01 August 2012 - 11:47 PM

If you have a pin registered gate which stops the film and use a monochrome Full Frame sensor with a R,G,B LED array to illuminate the film the pixel level alignment with be perfect as the film does not move. Both Line Array scanners and Full Frame scanners can give excellent results, the devil being in the details of how they are implemented.

-Rob-
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#5 Frank Vine

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Posted 03 August 2012 - 04:12 AM

There are 2 basic types of sensor being talked about here, a line scan sensor and a full frame sensor. Each has derivatives in architecture and process technology.
A line scan sensor is almost just what it says, a single line of pixels, although in reality some will actually have multiple lines for RGB and for intensity enhancement by having say 9 lines with the total charge being added up to increase sensitivity.

Full frame sensors fall into two main types, those with a rolling shutter and those with a global shutter. Although definitely not exclusively, the former shutter type are usually seen on CMOS sensors, the latter on CCD sensors.

Obviously with a line scanner, the image and sensor have to move relative to each other to effect the scan and they have to be tightly locked together otherwise there will be some vertical weaving.

For telecine, either full frame sensor shutter type can be suitable, but depends on the lighting design and the film advance mechanics. A rolling shutter kicks off exposure on the first line, then a few microseconds later on the second line, etc. The actual starting and stopping of the exposure will vary down the frame, so if the subject moves, image distortion will occur. For example, a rotating fan blade will appear to bend. A stationary frame in a film gate will not have this problem of course, but a continuous transport system will have an uneven exposure problem if used with a rolling shutter camera.

My own RGB lighting design uses a constant current through the LEDs during the exposure period. Actual exposure is adjusted by varying the RGB flash lengths in order to get close to full pixel saturation on all three channels so as to utilise the maximum useful dynamic range from the sensor. I use this method because if the current is varied through an LED the light colour frequency changes, making constant colour balanced exposure more difficult. Also it is easier to switch LEDs on and off with a varying period and fixed current than it is to vary the current with fixed period. Downside is that rolling shutter sensors are no good for this type of lighting design.

On the plus side for rolling shutter designs are that they are generally cheaper and have faster frame rates.

Sorry for the long post, but hope it gives some food for thought about how you want to achieve the lighting system and film transport and how this will link with the sensor type.
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